Ion-exchange membranes are used in electrodialysis and diffusion dialysis for a wide variety of applications such as concentration of seawater, desalination and removal of nitrate-nitrogen from underground brine for providing drinking water, desalination in a process for producing foods and concentration of active ingredients for a medicinal drug. Styrene-divinylbenzene based homogeneous ion-exchange membranes are typically useful in these applications. And owing to development of various technologies such as selectivity between monovalent and divalent ions, improvement in selectivity for a particular ion and lowering membrane resistance, the ion-exchange membranes have been improved to a level useful for industrial separation.
Generally, salts are often formed as by-products in a process for producing an organic compound in a field such as foods, medicinal drugs and agrochemicals as described above. Desalination of a raw liquid to be processed by electrodialysis using an ion-exchange membrane for separating salts contained in an organic substance encounters a problem of so-called organic fouling of a membrane where membrane performance is deteriorated due to adhesion of an organic foulant, particularly electrically-charged macromolecules (hereinafter, referred to as “macro-organic ions”) in a liquid to be processed.
As an ion-exchange membrane in which organic fouling is inhibited, an ion-exchange membrane preventing penetration of macro-organic ion into the membrane at its surface, and an ion-exchange membrane having high permeability of macro-organic ions were proposed. The high permeability of macro-organic ions can be easily achieved by formation of a loose membrane structure.
An ion-exchange membrane preventing penetration of macro-organic ions into a membrane has a thin layer, which is neutral or amphoteric or oppositely charged to an ionic group, on its surface. The more dense structure a membrane has and the higher molecular weight the macro-organic ion has, the more effective such an ion-exchange membrane is. A representative example of the ion-exchange membrane is an anion-exchange membrane wherein the surface of a resin membrane having an anion exchange group is modified with a sulfonic group having an opposite charge for preventing an organic anion from permeating the membrane (Patent Reference 1).
Patent Reference 1 has described a process for manufacturing an anion-exchange membrane, comprising sulfonating a membrane polymer having a functional group suitable for introducing an ionic group to partly introduce a sulfonic group to the functional group and then introducing an anion exchange group to the remaining functional group to convert the film polymer into an anion-exchange membrane, wherein the reaction conditions of sulfonation are selected such that an equivalent rate of a sulfonic group to the whole ionic groups is 0.05 to 20% and a ratio of direct-current resistance to alternating-current resistance in the anion-exchange membrane obtained is a threshold of a resistance ratio or less. It has been also described that an anion-exchange membrane prepared by the process exhibits anti-organic fouling and can maintain a higher current efficiency and a lower electric resistance. It has been further described that an anion-exchange membrane having anti-organic fouling property can be provided by convenient means. It, however, has a problem that an electric resistance of the ion-exchange membrane (membrane resistance) substantially increases due to the oppositely charged layer formed in the surface of the resin film.
Patent Reference 2 has described an ion-exchange membrane wherein a polymer chain having an ionic group samely charged to the above ionic group is bound to the surface of an ion-exchange membrane made of a resin containing an ionic-group. It has been described that such an ion-exchange membrane can have excellent anti-organic fouling property without increase in a membrane resistance and can maintain high performance in electrodialysis of a system involving a macro-organic substance for a long period. However, there remains the need for an ion-exchange membrane exhibiting more improved anti-organic fouling and a lower membrane resistance.
Patent Reference 3 has described an ion-exchange membrane wherein a polyether compound containing a polyalkyleneglycol chain is fixed in the surface and/or inside of the membrane. It is believed that the polyether compound present in the surface or the inside of the ion-exchange membrane prevents macro-organic ions and the like from directly contacting the ionic group, so that adsorption of macro-organic ions and the like by the ion-exchange membrane is minimized and consequently anti-organic fouling property is improved. It has been also described that the polyether compound can substantially inhibit increase in an electric resistance of the ion-exchange membrane owing to hydrophilicity derived from the alkyleneglycol chain. However, there remains the need for an ion-exchange membrane exhibiting more improved anti-organic fouling property and a lower membrane resistance.
Patent Reference 4 has described an ion-exchange membrane wherein voids in a microporous membrane are filled with an ion-exchange resin and in pores present at least in one side of the microporous membrane, the ion-exchange resin is exposed at a level lower than the surface of the microporous membrane. Furthermore, Patent Reference 5 has described an ion-exchange membrane for electrodialysis consisting of a microporous membrane having penetrating pores filled with an ion-exchange resin, wherein in the membrane surface, a pore size is 5 μm or less and the pores occupy 3 to 60% of the whole area, and a thickness of the membrane is 15 to 120 μm. It has been described that such an ion-exchange membrane can exhibit excellent anti-organic fouling without deteriorating basic properties such as membrane resistance and ion selectivity. A process for manufacturing such a membrane, however, has a problem that it requires advanced technology and troublesome manufacturing steps.
A charge-mosaic membrane is an ion-exchange membrane comprised of cation-exchange domains and anion-exchange domains which are alternately aligned in a parallel manner and each of which penetrates the membrane from one side to the other side. This unique charge structure can promote permeation of low-molecular-weight ions in a given solution without requiring an external current. When positive charge domains and negative charge domains are aligned in a mosaic manner, an electric circuit in which salt solution positioned on both sides of the membrane act as resistances is formed because these regions have a mutually opposite potential direction. When cations and anions are supplied to the circuit through the negative and the positive charge domains like a current applied to it, respectively, a circulating current is generated, so that salt transport is promoted. It means that a charge-mosaic membrane itself has an inherent mechanism for causing ion transport in contrast to an ion-exchange membrane with a single fixed charge which requires an external current.
There have been reported charge-mosaic membranes produced by various processes. Patent Reference 6 has described a method for desalination an organic compound using a charge-mosaic membrane prepared utilizing a microphase separation phenomenon in a block copolymer. However, a method for producing a charge-mosaic membrane utilizing microphase separation phenomenon of a block copolymer requires very troublesome and advanced technique such as modification of a particular site in a block copolymer, and is so costly that a charge-mosaic membrane cannot be easily produced in a large size or at low cost.
Patent Reference 7 has described a process for producing a charge-mosaic membrane, comprising mixing a membrane-forming polymer, a solvent capable of dissolving the membrane-forming polymer, a cation-exchange resin and an anion-exchange resin to prepare a homogeneous polymer dispersion in which the cation-exchange resin and the anion-exchange resin are dispersed in a polymer solution; coating and extending the polymer dispersion to a substrate; drying it to be solidified; removing a solvent from the film thus obtained and washing the membrane. A charge-mosaic membrane prepared by the process exhibits increase in an amount of permeating salts with increase in a pressure as measured in a piezodialysis experiment. However, in this charge-mosaic membrane, a membrane matrix is not chemically bonded to the ion-exchange resin, and thus, in an interface between them, water and/or a neutral solute leak. High salt permselectivity cannot be, therefore, achieved.
Patent Reference 8 has described a process for producing a charge-mosaic membrane consisting of cationic polymer domains and anionic polymer domains wherein in a crosslinked continuous phase formed by an ionic (either cationic or anionic) polymer, a polymer at least having ionicity opposite to the continuous-phase forming polymer is dispersed as crosslinked particles with an average particle size of 0.01 to 10 μm. The process comprises forming a membrane using a dispersion prepared by dispersing, in a solution of an either ionic polymer forming the continuous phase in the membrane, spherical polymer particles with at least ionicity opposite to the continuous-phase forming polymer; then crosslinking at least the continuous phase in the membrane; and then immersing the membrane in water or an aqueous solution. For a membrane prepared by this process, a domain size and a thickness can be easily regulated and as the most advantageous feature, a membrane with a large area can be relatively easily prepared. This manufacturing process has a problem that the necessity of preparing polymer particles with a small average particle size requires advanced technique and a longer period. Furthermore, since the charge-mosaic membrane thus prepared contains a microgel with a high water content, it exhibits quite poor pressure resistance. In particular, it has a structure in which interfacial adhesion between the membrane matrix and the positive/negative microgel is insufficient. Therefore, a charge-mosaic membrane exhibiting higher electrolyte permeability and mechanical strength is inadequate. Therefore, although the membrane can be used as a membrane for diffusion dialysis, it cannot be used as a membrane for piezodialysis or exhibits extremely poor durability.
Non-patent Reference 1 has described a charge-mosaic membrane prepared by a lamination method. In this lamination method, cation-exchange membranes are prepared from polyvinyl alcohol and a polyanion, and anion-exchange membranes are prepared from polyvinyl alcohol and a polycation, respectively, and these are alternately laminated via polyvinyl alcohol as an adhesive to form a laminated charged block. The block is cut by a laboratory cutter perpendicularly to the lamination plane and crosslinked to give a laminated charge-mosaic membrane with a thickness of about 150 μm. It is described that a laminated charge-mosaic membrane thus prepared has a KCl-salt flux (JKCl) of 3.0×10−9 mol·cm−2·S−1 and an electrolyte permselectivity (α) of 2300, which means that the membrane is very permselective. A tensile strength is 5.7 MPa in a direction parallel to a charged layer while being 2.7 MPa in a vertical direction, indicating that the membrane can be used for diffusion dialysis but must be stronger for piezodialysis applications. Furthermore, its salt permeation flux is so small that the membrane cannot provide adequate dialysis performance.
Non-patent Reference 2 has described a charge-mosaic membrane prepared by a polymer blend method using polyvinyl alcohol as a membrane matrix. In the polymer blend method, to an aqueous solution of a modified PVA polyanion containing polyvinyl alcohol and a vinyl compound having an itaconic group as 2 mol % copolymerization composition is added hydrochloric acid to acidify the solution for preventing dissociation of hydrogen ion from a carboxyl moiety in an itaconic group. To the solution are added polyvinyl alcohol and an aqueous solution of polyallylamine hydrochloride to prepare an aqueous solution of blended polymers. This solution is cast on, for example, a glass plate to form a film, which is then chemically crosslinked to provide a charge-mosaic membrane. It is described that a charge-mosaic membrane thus obtained has a KCl-salt flux (JKCl) of 1.7×10−8 mol·cm−2·s−1 and an electrolyte permselectivity (α) of 48, which is relatively higher, but a further higher electrolyte permselectivity (α) is required. Furthermore, there is a problem that salt permselectivity is reduced in an acidic solution.